J Clust Sci (2009) 20:205–211 DOI 10.1007/s10876-008-0220-7 ORIGINAL PAPER
Electronic Structure of EuMo6Se8 Studied by X-Ray Absorption Spectroscopy K. Asokan Æ O. Pen˜a Æ L. Le Polles Æ J. C. Jan Æ J. W. Chiou Æ W. F. Pong
Received: 13 September 2008 / Published online: 19 November 2008 Ó Springer Science+Business Media, LLC 2008
Abstract The rare-earth based molybdenum chalcogenides, REMo6Se8 (RE = rare-earth metals) have been extensively studied because of their unique crystal structure based on Mo6Se8 clusters and their outstanding properties involving coexistence of superconductivity and magnetism. Among all these compounds, Ce and Eu based chalcogenides are magnetic and non-superconductors and possess many novel properties. Understanding their electronic structure is likely to provide valuable information about these materials. We employ X-ray absorption near-edge structure (XANES) spectroscopy at Mo and Se K-edges of EuMo6Se8 to identify the local environment respectively around Mo and Se ions and XANES spectra at L3edge of Eu ion to identify their valence state. Results from this study demonstrate that Se ions in EuMo6Se8 are in two inequivalent sites and the valency of Eu is divalent.
K. Asokan (&) Inter-University Accelerator Centre, Aruna Asaf Ali Marg, New Delhi 110 067, India e-mail:
[email protected] O. Pen˜a L. Le Polles Sciences Chimiques de Rennes, UMR 6226, Universite´ de Rennes 1, Rennes, France L. Le Polles ENS de Chimie de Rennes, Rennes, France J. C. Jan National Synchrotron Radiation Research Center, Hsinchu 300, Taiwan, ROC J. W. Chiou Department of Applied Physics, National University of Kaohsiung, Kaohsiung 811, Taiwan, ROC W. F. Pong Department of Physics, Tamkang University, Tamsui 251, Taiwan, ROC
123
206
K. Asokan et al.
Keywords Chevrel phase compounds Chalcogenides X-ray absorption spectroscopy Eu valence
Introduction The ternary molybdenum chalcogenides commonly known as Chevrel phases compounds have been extensively investigated for their superconducting properties, thermoelectric and cathode materials [1–3]. These compounds are represented with a general formula MMo6X8, in which X = S, Se or Te, and M can be a simple or transition metal atom, or a rare-earth element [1, 2]. The building blocks of their crystal structure are binary Mo6X8 clusters and it has been recognized that bonding in Chevrel phases is mainly due to Mo and X atoms, with important Mo d- Mo d intracluster interactions and Mo d-X p covalent mixing [2]. The rare-earth (RE) based Chevrel type phases, REMo6Se8, exhibit coexistence of superconductivity and magnetic ordering. With a lanthanide element, magnetic interactions may lead to an ordered sublattice without destroying superconductivity [4]. With the exception of RE = Ce, and Eu, all other ternary compounds are superconductors with transition temperatures varying from 5.5 to 11 K [4–6]. It was found that the Eu based compound differ enormously from other lanthanides because of electronic instabilities triggered by a structural transition [7]. For instance EuMo6S8 metallic and superconducts at 11 K, for pressures higher than 13 kbar. But for the selenide compound, almost no change in the transport properties is observed in EuMo6Se8 even under high pressure [7, 8]. Extensive studies have been carried out in this compound to understand the properties of EuMo6Se8 using various experimental techniques [9]. X-ray absorption spectroscopy provides chemically selective information like valency and the bonding of the absorbing ion [10]. Present study uses X-ray absorption near-edge structure (XANES) spectroscopy to understand the local electronic structure of EuMo6Se8 especially at Mo, Se and Eu sites. For comparison purposes, the superconducting and magnetic NdMo6Se8 compound is briefly reported in this work.
Experimental The ternary phases EuMo6Se8 and NdMo6Se8 were prepared by solid-state reactions of the molybdenum powder and the binary selenides MoSe2 and Eu (or Nd)Sey. Stoichiometric quantities were compacted into pellets, placed inside outgassed alumina crucibles and sintered at 1200 °C for three days, after two 24-h presintering stages at 900 and 1100 °C, respectively. Neither impurities nor secondary unreacted phases were detected at this stage within the sensitivity (*5%) of the X-ray powder diffraction technique [9]. X-ray absorption near-edge structure at Mo K-edge measurements were done at Taiwan beamline, SPRING-8, Japan in both transmission and fluorescence mode and Se K- and Eu L3-edge at Wiggler beamline NSRRC, Taiwan at room temperature by following standard procedures.
123
Electronic Structure of EuMo6Se8 Studied by XANES Spectroscopy
207
Results and Discussion From the X-ray diffraction measurements, the crystal structure of EuMo6Se8 is rhombohedral with space group R-3 (n° 148), isotypical to Chevrel phases containing large cations [9, 11]. It is composed of Mo6Se8 blocks, in which an octahedral cluster Mo6 of 6 molybdenum atoms is situated inside an almost regular cubic cage of 8 chalcogen atoms. The Mo6Se8 unit is rotated by about 25° with respect to the three-fold axis inside a rhombohedral cationic lattice. The rare-earth atom located at the origin of the rhombohedral unit cell is surrounded by eight chalcogen atoms belonging to eight different Mo6Se8 units. Figure 1 shows the crystal structure composed of the octahedral cluster of Mo6 atoms inside a Se8 pseudo cubic cage of the Mo6Se8 unit. Each Mo atom is situated in a pyramidal environment and bonded to one Se (2) atom located in the three fold axis and to four Se(1) atoms (one of which belongs to an adjacent Mo6Se8 unit). The normalized XANES at Mo K-edge of Chevrel phases are shown in Fig. 2. The XANES spectra in principle should be characteristic of oxidation states and the covalence between the metal-ligand bonding [12]. The XANES is dominated by the pre-edge feature marked in the figure as p (for MoO3), which corresponds to a tetrahedral environment surrounded by four ligands. Absence of pre-edge peak implies an octahedral environment of Mo cations in these compounds. Mo K-edge does not show variation with Eu, Nd inclusions in the chalcogenides implying absence of valence change in Mo [13, 14]. These results show that the local structure of Mo K-edge does not present any appreciable variation between EuMo6Se8 and NdMo6Se8. Any determination of the exact valence of Mo in Eu and Nd compounds is complicated as these are clusters without identifying suitable reference compounds for them. However, one can observe that valence of Mo in Eu
Fig. 1 Structure of Chevrel phase compounds: Note that the Mo ion is deep inside the Se clusters
123
208
K. Asokan et al.
Fig. 2 Normalized XANES spectra at Mo K-edge of compounds of chalcogenides along with reference spectra of MoO3
Mo K-edge A1
NdMo6Se8
Intensity (arb units)
EuMo6Se8
MoO3
p
19950
19975
20000
20025
20050
20075
Photon Energy (eV)
and Nd based compounds are the same. In ternary phases, the metallic species M remain weakly coupled to the rest of the structure, with only secondary effects on the electronic properties, modifying the distance between the Mo6X8 clusters, and therefore Mo 4d bandwidth, and providing electrons for band filling. This implies important consequence on the physical properties [1, 2]. In order to understand the chemical nature of Se ions, we measured Se K-edge in these compounds along with Mo6Se8 compounds and it is shown in Fig. 3. As evident, the local structure of Se K-edge is altered with the introduction of Nd and Eu ions in the voids of the crystal structure when compared to Mo6Se8. The mainedge spectral signatures marked A2, and B2 arise due to multiple-scattering effects of the ejected photoelectron wave from the central absorbing atom among its nearest and next-nearest neighbours [15–17]. These effects also are related to different overlapping of the mixed orbitals and consequently due to geometry of the absorbing atom, Se. Apart from this, we can also infer that Se 4p states are highly delocalized and extended over at least two near neighbors of Se ions. From structural consideration, it is known that this compound contains two types of sites marked in Fig. 1 as Se(1) and Se(2). If these two Se ions are geometrically equivalent, one would expect the photoelectron scattering resonances, i.e., intensities of peaks A2, and B2 should have been of same strength due to the multiple scattering processes involved in the XANES region [16, 17]. Since
123
Electronic Structure of EuMo6Se8 Studied by XANES Spectroscopy Fig. 3 Normalized XANES spectra at Se K-edge of compounds of EuMo6Se8, NdMo6Se8 and Mo6Se8
209
Se K-edge
Intensity (arb units)
A2 B2
EuMo6Se8
NdMo6Se8
Mo6Se8
12620
12640
12660
12680
12700
Photon Energy (eV)
intensities of these peaks are different in EuMo6Se8, it thus shows that the Se ions are in geometrically frustrated environment compared to NdMo6Se8. If XANES spectra are interpreted in the multiple-scattering approach, the sensitivity of XANES both for the geometrical arrangement of atoms as well as for the local electronic structure, is evident [16, 17]. The X-ray absorption of L3-edge of rare-earth ions is the result of electrons from the deep 2p core level to the 5d band [18, 19]. The energy of this transition shifts with the valence state of the absorber ion and this shift occurs because the transfer of an electron from the 2p shell inside the 4f shell depends on the 4f charge. This edge has been widely used mainly to get information about valency of the rare-earth ions. In the case of two different states of the valence or in the case of configuration fluctuations, the spectra will have a complicated composite shape. Figure 4 shows the normalized XANES spectra at Eu L3-edge of EuMo6Se8 and EuMnO3. As evident from these spectra, the peak at *6975 eV corresponds to divalent state of Eu ion and *6983 eV that of trivalent state. In EuMnO3, Eu is in a trivalent state, which forms as reference spectrum. The binding energy difference of Eu L3 between EuMo6Se8 and EuMnO3 is about 8 eV and this implies that EuMo6Se8 is in pure divalent state [18, 19]. These spectra also directly show that there is no valence fluctuation in this chalcogenide. Above spectroscopic result is consistent with the
123
210
EuL 3-Edge
2+
Eu
3+
Eu
EuMo6Se8 EuMnO3
Intensity (arb. units)
Fig. 4 Normalized XANES spectra at Eu L3-edge of EuMnO3 and EuMo6Se8. Note that there is a shift of *8 eV between 2? and ?3 valency states of Eu ions
K. Asokan et al.
6945
6960
6975
6990
7005
7020
7035
Photon Energy (eV)
magnetic measurements [9] wherein their magnetization saturation results confirm strict divalent character of the Eu ion.
Conclusions Present study using XANES spectroscopy shows that there is no change in the local electronic structure of Mo ions when compared to superconducting and nonsuperconducting chalcogenides but there is significant change around Se ion in EuMo6Se8 compared to superconducting NdMo6Se8 ions. The valence of Eu is in divalent state which is consistent other results including magnetic measurements. Acknowledgments One of the authors (KA) wishes to thank French Embassy for their kind support and encouragement which in fact made him to present this work and to the IWTMC organizers for the invitation to participate at the conference. The author (WFP) would like to thank the National Science Council of Taiwan for financially supporting this research under Contract No. NSC96-2112-M032-012MY3.
References 1. 2. 3. 4. 5.
R. Chevrel, M. Sergent, and J. Prigent (1971). J. Solid State Chem. 3, 515. Ø. Fischer (1978). Appl. Phys. 16, 1. E. Levi, A. Mitelman, O. Isnard, M. Brunelli, and D. Aurbach (2008). Inorg. Chem. 47, 1975. O. Pen˜a and M. Sergent (1989). Prog. Solid State Chem. 19, 165. F. Le Berre, O. Pena, C. Hamard, R. Horyn, and A. Wojakowski (1997). J. Alloys Compd. 262–263, 406. 6. F. Le Berre, O. Pen˜a, C. Perrin, M. Sergent, R. Horyn, and A. Wojakowski (1998). J. Solid State Chem. 136, 151. 7. C. W. Chu, S. Z. Huang, J. H. Lin, R. L. Meng, M. K. Wu, and P. H. Schmidt (1981). Phys. Rev. Lett. 46, 276. 8. C. Rossel, H. W. Meul, A. Junod, R. Baillif, Ø. Fischer, and M. Decroux (1983). Solid State Commun. 48, 431.
123
Electronic Structure of EuMo6Se8 Studied by XANES Spectroscopy
211
9. F. Le Berre, O. Pen˜a, C. Perrin, J. Padiou, R. Horyn, and A. Wojakowski (1998). J. Alloys Compd. 280, 85. 10. J. Stohr, NEXAFS Spectroscopy (Springer, Berlin, 1992). 11. O. Pen˜a, C. Geantet, H. Schmitt, F. Le Berre, and C. Hamard (1999). Solid State Sci. 1, 577. 12. G. Pfeiffer, J. J. Rehr, and D. E. Sayers (1995). Phys. Rev. B 51, 804. 13. P. P. Vaishnava, C. W. Kimball, J. L. Matykiewicz, F. Y. Fradin, G. K. Shenoy, and P. A. Montano (1986). Phys. Rev. B 34, 4599. 14. L. M. Reilly, G. Sankar, and C. R. A. Catlow (1999). J. Solid State Chem. 148, 178. 15. M. Popescu, F. Sava, N. Aldea, Y. Xie, T. Hu, T. Liu, and M. Leonovici (2005). Chalcogenide Lett. 2, 71. 16. K. Asokan, C. L. Dong, C. W. Bao, H. M. Tsai, J. W. Chiou, C. L. Chang, W. F. Pong, P. Duran, C. Moure, and O. Pen˜a (2007). AIP Conf. Proc. 879, 1659. 17. A. L. Ankundinov, B. Ravel, J. J. Rehr, and S. D. Conradson (1998). Phys. Rev. B 58, 756. 18. A. Mitsuda, S. Fukuda, Y. Nakanuma, N. Ietaka, Y. Isikawa, and T. Yamamoto, in Photon Factory Activity Report 2002#20 Part B (2003), p. 101. 19. M. R. Antonio, L. Soderholm, G. Jennings, L. C. Francesconi, M. Dankova, and J. Bartis (1998). J. Alloys Compd. 275–277, 827.
123
